1. Field of the Invention
The present invention relates to an MRI apparatus for generating a MRI (magnetic resonance imaging) image using a binomial pulse for exciting a species of nuclear spin.
2. Description of the Related Art
An MRI apparatus is a unit with an imaging process for reconstructing the MRI image of a cross section in an object based on an MR signal generated by magnetically exciting a species of nuclear spins of the object in the static magnetic field with a high frequency signal at Larmor frequency.
When generating an MRI image based on MR signal received by water (free water) excitation with the MRI apparatus, a fat suppression technology (to reduce collection of MR signal from fat) is necessary to avoid chemical shift artifact caused by fat (bound water) in cross section. Meanwhile, when generating an MRI image based on MR signal received by fat excitation, water suppression technology (to reduce collection of MR signal from water) is necessary to avoid chemical shift artifact caused by water in the cross section. Generally, processes using various types of pulse sequences have been well known to perform the aforementioned fat suppression and water suppression. For example, signal from fat or water may be suppressed using a frequency selective excitation pulse. The process employs an RF wave envelope (binomial pulse) with a flip angle ratio of 1-2-1 series or 1-3-3-1 series of water or fat as the frequency selective excitation pulse.
However, in order to improve properties of water excitation and fat excitation with generally employed technology, both an echo time (TE) and a repetition time (TR) are prolonged, and accordingly, the entire imaging time becomes long, which may exert stress to the object.
In the generally employed art, the segregation property between water and fat sometimes cannot be further improved owing to weakened susceptibility because the longer the pulse length becomes, the lower the S/N ratio becomes.